The present disclosure relates to a method of ring-opening polymerization, and related compositions and articles.
Biodegradable polymers offer a viable alternative to commodity plastics in a number of bulk applications. Among biodegradable polymers, the best known are poly(hydroxyalkanoates) (PHAs), which can be produced on a large scale through bacterial fermentation. The most common representative of this family of biopolymers, poly-([R]-3-hydroxybutyrate) with isotactic structure, is synthesized by a variety of bacteria as a reserve energy source and possesses a remarkable feature of being totally biodegradable in various environments. The lack of commercialization of the initially promising bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymers has been generally attributed to the high investment for the fermentation and product recovery processes on a large scale, and to the cost of the substrates.
Ring-opening polymerization (ROP) of beta-substituted propiolactones offers another approach for producing PHAs. Most efforts to date have focused on the ROP of beta-butyrolactone (b-BL) to make poly(3-hydroxybutyrate) (PHB). Unlike bacteria-mediated polymerization, which gives only isotactic PHB, controlled ROP of b-BL allows access to a variety of PHB microstructures. For example, racemic b-BL has been polymerized to make atactic PHB as well as PHB enriched in isotactic and syndiotactic diads. Optically pure b-BL can also be polymerized to make highly isotactic PHB.
With the exception of recently reported distannoxane and alkylzinc alkoxide catalysts, most systems previously studied for the ROP of b-BL are extremely slow and/or are not capable of producing high molecular weight poly(3-hydroxybutyrate) (Mn>100,000) in a controlled manner. Other drawbacks include the toxicity of metallic initiator residue and unwanted byproducts such as crotonates (alpha, beta-unsaturated esters formed by elimination of a carboxylate from the polymer backbone).
Thus, an ongoing challenge exists to improve the ring-opening polymerization of cyclic esters for the large scale production of biodegradable polymers.